The present invention relates to optical communications systems. More particularly, it concerns optical communications systems for high density communications between a plurality of terminal devices.
In optical fiber communications systems, it is known to transmit information using both digital and analog techniques. In digital systems, an optical source, such as a light emitting diode or a laser diode, is directly modulated by a base band signal drive current with the pulsed optical output launched onto an optical path for propagation to a photodetector, such as a PIN diode or an avalanche photodiode (APD), which directly demodulates the optical energy to provide a recovered base band signal output. In such systems, the bandwidth is limited by the bandwidth of the optical source and the photodetector, as well as the optical communications path. In analog systems, the output of a carrier source can be modulated in a time varying manner and propagated to a photodetector for demodulation. In the simplest arrangement, the optical carrier can be subjected to envelope modulation and directly demodulated in a nonlinear photodetector. In more sophisticated arrangements, such as those involving wavelength or phase modulation, more sophisticated modulating and demodulating devices are required to recover the information content.
In the radio frequency portion of the electromagnetic spectrum, it is common to modulate a source carrier with information for propagation to a receiving device. Demodulation of a received signal can be effected by providing a local oscillator at the receiving device which generates a local oscillator signal which is mixed with the received signal. The local oscillator output can be offset in frequency from that of the received signal to produce a heterodyned intermediate frequency which can be subsequently demodulated or a local oscillator signal which is frequency and phase matched with the received signal to effect synchronous homodyne detection. Regardless of the type of detection utilized, the local oscillator source at the receiving device must be stable and accurately tunable.
While accurate, tunable, and highly stable local oscillator sources are readily available in the radio frequency spectrum, there are presently no corresponding sources available in the optical portion of the electromagnetic spectrum. For example, gas lasers represent a class of potential local oscillator sources because of their inherent stability and accuracy. However, gas lasers are not readily tunable and their unique power supply requirements, physical size, and cost make them unsuitable for use as local oscillators in terminal devices in a large communication system. In contrast to gas lasers, laser diodes can be wavelength controlled by varying their drive current. However, their output is not necessarily wavelength stable and is subject to long term frequency drift as a function of operating current, temperature, and to some extent, operating life. In addition, the output frequency of laser diodes is subject to short term variations as a result of optical feedback and longitudinal mode jumping in the resonant cavity such that the frequency spectra at the output varies with time about an average value. In the absence of large frequency shifts caused by mode hopping, the remanent short term "jitter" is typically 10-100 MHz and is a function of changes in the population inversion level, the index of refraction, and temperature. This short term jitter limits the channel density in wavelength multiplexed systems since channel bandwidth for filtering or coherent detection can not be narrower than the spectral frequency distribution of the laser diode output, otherwise power is lost to the detection system.
Accordingly, present efforts to develop optical communication systems which utilize a local optical source at the terminal devices, to either assist in the generation of a modulated carrier for launching onto an optical pathway or for detection purposes, is limited by presently available devices.